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ABSTRACT: With the advent of pure-spin-current sources, spin-based electronic (spintronic) devices no longer require electrical charge transfer, opening new possibilities for both conducting and insulating spintronic systems. Pure spin currents have been used to suppress noise caused by thermal fluctuations in magnetic nanodevices, amplify propagating magnetization waves, and to reduce the dynamic damping in magnetic films. However, generation of coherent auto-oscillations by pure spin currents has not been achieved so far. Here we demonstrate the generation of single-mode coherent auto-oscillations in a device that combines local injection of a pure spin current with enhanced spin-wave radiation losses. Counterintuitively, radiation losses enable excitation of auto-oscillation, suppressing the nonlinear processes that prevent auto-oscillation by redistributing the energy between different modes. Our devices exhibit auto-oscillations at moderate current densities, at a microwave frequency tunable over a wide range. These findings suggest a new route for the implementation of nanoscale microwave sources for next-generation integrated electronics.
Nature Material 10/2012; · 32.84 Impact Factor
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ABSTRACT: Spin currents--the flow of angular momentum without the simultaneous transfer of electrical charge--play an enabling role in the field of spintronics. Unlike the charge current, the spin current is not a conservative quantity within the conduction carrier system. This is due to the presence of the spin-orbit interaction that couples the spin of the carriers to angular momentum in the lattice. This spin-lattice coupling acts also as the source of damping in magnetic materials, where the precessing magnetic moment experiences a torque towards its equilibrium orientation; the excess angular momentum in the magnetic subsystem flows into the lattice. Here we show that this flow can be reversed by the three-magnon splitting process and experimentally achieve the enhancement of the spin current emitted by the interacting spin waves. This mechanism triggers angular momentum transfer from the lattice to the magnetic subsystem and modifies the spin-current emission. The finding illustrates the importance of magnon-magnon interactions for developing spin-current based electronics.
Nature Material 07/2011; 10(9):660-4. · 32.84 Impact Factor
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ABSTRACT: The transition from stationary to chaotic spin-wave soliton trains has been observed. The experiment utilized cw excitation of envelope solitons through self-modulation instability of spin waves. By increasing the spin-wave power, the secondary self-modulation instability succeeded the primary modulation instability, resulting in after-modulation of the soliton train amplitude. Further increase of the spin-wave power led to development of the higher-order instabilities, resulting in formation of the chaotic soliton train.
Physical Review Letters 01/2011; 106(1):017201. · 7.37 Impact Factor
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ABSTRACT: We demonstrate that microwave pumping of a spin-torque nano-oscillator can lead to transfer of the spin-wave energy generated by the oscillator into a mode with frequency given by the difference between the pumping frequency and that of the auto-oscillation. The decay length of spin waves at the combination frequency is significantly increased, while the directionality of emission is preserved. The observed phenomenon provides a route for the implementation of nanosized spin-wave emitters with controllable emission characteristics for applications in next-generation microwave electronic devices.
01/2011; 060406.
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ABSTRACT: Dynamics induced by spin-transfer torque is a quickly developing topic in modern magnetism, which has initiated several new approaches to magnetic nanodevices. It is now well established that a spin-polarized electric current injected into a ferromagnetic layer through a nanocontact exerts a torque on the magnetization, leading to microwave-frequency precession detectable through the magnetoresistance effect. This phenomenon provides a way for the realization of tunable nanometre-size microwave oscillators, the so-called spin-torque nano-oscillators (STNOs). Present theories of STNOs are mainly based on pioneering works predicting emission of spin waves due to the spin torque. Despite intense experimental studies, until now this spin-wave emission has not been observed. Here, we report the first experimental observation and two-dimensional mapping of spin waves emitted by STNOs. We demonstrate that the emission is strongly directional, and the direction of the spin-wave propagation is steerable by the magnetic field. The information about the emitted spin waves obtained in our measurements is of key importance for the understanding of the physics of STNOs, and for the implementation of coupling between individual oscillators mediated by spin waves. Analysis shows that the observed directional emission is a general property inherent to any dynamical system with strongly anisotropic dispersion.
Nature Material 10/2010; 9(12):984-8. · 32.84 Impact Factor
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ABSTRACT: We have studied experimentally the interaction of nonlinear packets of spin waves with strongly localized nonuniformities of the static magnetic field representing magnetic potential barriers and wells. We have found that the nonlinearity in the system causes a noticeable modification of this interaction in comparison to the linear case. The strongest modification is observed under conditions where spin-wave envelope solitons are formed. Our findings show that for the case of potential barriers the solitons demonstrate an enhanced tunnel-ing, whereas for potential wells they show an enhanced reflection. Moreover, the nonlinear enhancement of the interaction was found to be stronger for potential wells, which was associated with its resonant character.
04/2008;
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ABSTRACT: Modifications of spatial distributions of dynamic magnetization corresponding to spinwave eigenmodes of magnetic squares subjected to a strong microwave excitation field have been studied experimentally and theoretically. We show that an increase of the excitation power leads to a nonlinear generation of long-wavelength spatial harmonics caused by the nonlinear cross coupling between the eigenmodes. The analysis of the experimental data shows that this process is mainly governed by the action of the nonlinear spin-wave damping. This conclusion is further supported by the numerical calculations based on the complex Ginzburg-Landau equation phenomenologically taking into account the nonlinear damping. Comment: 23 pages, 6 figures
11/2007;
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ABSTRACT: We have studied the tunneling of spin-wave pulses through a system of two closely situated potential barriers. The barriers represent two areas of inhomogeneity of the static magnetic field, where the existence of spin waves is forbidden. We show that for certain values of the spin-wave frequency corresponding to the quantized spin-wave states existing in the well formed between the barriers, the tunneling has a resonant character. As a result, transmission of spin-wave packets through the double-barrier structure is much more efficient than the sequent tunneling through two single barriers.
Physical Review Letters 10/2007; 99(12):127204. · 7.37 Impact Factor
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ABSTRACT: Using low-loss dielectric magnetic films in combination with space-resolved Brillouin light scattering spectroscopy we have studied nonlinear modification of eigenmode spatial distributions in saturated magnetic squares. We have found that, as the angle of magnetization precession increases, the eigenmode spatial distributions experience significant qualitative changes due to a nonlinear coupling between forming them standing spin waves. We show that the found nonlinear eigenmodes cannot be described by means of the linear theoretical approach even qualitatively.
Physical Review Letters 05/2007; 98(15):157203. · 7.37 Impact Factor
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ABSTRACT: The author numerically conducted micromagnetic modeling to optimize the shapes of patterned magnetic thin-film elements applicable to double-contact gigahertz-frequency-range nano-oscillators. Spin waves emitted from one of two foci in the optimized shapes of nanoscale elements can be collected in phase at the other focal point via coherent and constructive interference of all the spin waves propagating directly between the two foci and reflected at the different boundary positions of the elements. This work provides promising ways to design the shapes of patterned magnetic thin films with in-plane and out-of-plane magnetizations applicable to phase-locked gigahertz-frequency-range nanooscillators with optimized spin-wave coupling.
Applied Physics Letters 02/2007; 90(8):083114-083114-3. · 3.84 Impact Factor
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ABSTRACT: The radiation of spin waves by rectangular spin-valve elements into a surrounding magnetic film has been studied experimentally using a novel micro-focus Brillouin light scattering setup allowing for spatially resolved measurements of the intensity of dynamic magnetization with a resolution better than 300 nm . The investigated elements have lateral dimensions of 1.3×2.3 μ m <sup>2</sup> and consist of a 10 nm thick Co <sub>80</sub> Fe <sub>20</sub> and a 5 nm thick Ni <sub>81</sub> Fe <sub>19</sub> layer separated by a 4 nm thick Cu spacer layer. It was found, that the elements radiate spin waves at frequencies corresponding to their laterally quantized spin-wave eigenmodes. Two-dimensional distributions of intensities of spin waves radiated by different eigenmodes were recorded outside the element. It was shown, that the radiation patterns consist of several rays intersecting each other and forming spots where the amplitude of variable magnetization locally increases. A theoretical model qualitatively explaining the observed radiation patterns has been suggested.
Journal of Applied Physics 06/2005; · 2.17 Impact Factor
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ABSTRACT: Quantized spin-wave eigenmodes in single, 16 nm thick and 0.75 to 4 mum wide square permalloy islands with a fourfold closure domain structure have been investigated by microfocus Brillouin light scattering spectroscopy and time resolved scanning magneto-optical Kerr microscopy. Up to six eigenmodes were detected and classified. The main direction of the spin-wave quantization in the domains was found to be perpendicular to the local static magnetization. An additional less pronounced quantization along the direction parallel to the static magnetization was also observed.
Physical Review Letters 03/2005; 94(5):057202. · 7.37 Impact Factor
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ABSTRACT: Dynamic magnetic properties of a single micrometer-sized magnetic element consisting of a permalloy and a partially patterned CoFe layer separated by an intervening Cu spacer layer have been studied by means of a micro-focus Brillouin light scattering setup, which allows for local measurements of the magnetization dynamics on the submicrometer scale. It is shown that quantized spin-wave modes excited in the magnetic element act as radiation sources for spin waves in the surrounding magnetic film. It is found that the intensities of spin waves excited by different quantized modes follow different distance laws when traveling away from the region of excitation.
Applied Physics Letters 10/2004; 85(14):2866-2868. · 3.84 Impact Factor
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ABSTRACT: Solitons are large-amplitude, spatially confined wave packets in nonlinear media. They occur in a wide range of physical systems, such as water surfaces, optical fibres, plasmas, Bose-Einstein condensates and magnetically ordered media. A distinguishing feature of soliton behaviour that is common to all systems, is that they propagate without a change in shape owing to the stabilizing effect of the particular nonlinearity involved. When the propagation path is closed, modes consisting of one or several solitons may rotate around the ring, the topology of which imposes additional constraints on their allowed frequencies and phases. Here we measure the mode spectrum of spin-wave solitons in a nonlinear active ring constructed from a magnetic ferrite film. Several unusual symmetry-breaking soliton-like modes are found, such as 'Möbius' solitons, which break the fundamental symmetry of 2pi-periodicity in the phase change acquired per loop: a Möbius soliton needs to travel twice around the ring to meet the initial phase condition.
Nature 12/2003; 426(6963):159-62. · 36.28 Impact Factor
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ABSTRACT: The propagation of spin-wave beams in ferromagnetic stripes has been investigated experimentally by means of spatially resolved Brillouin light scattering spectroscopy. It is shown that in the case of low-amplitude spin waves the diffraction and the transverse confinement of spin waves lead to oscillations of the beam width. The authors found that with increasing spin-wave amplitude the interplay between the linear diffraction and the repulsive spin-wave nonlinearity results in a suppression of the oscillations of the beam width and in its transverse stabilization. © 2006 American Institute of Physics. The fast development in magnetic integrated technology demands a deep understanding of high-frequency magnetic dynamics in thin magnetic films and nanostructured mag-netic elements. Recently spin waves in such systems have been intensively studied see, e.g., Refs. 1–5 and references therein by means of a number of experimental techniques including Brillouin light scattering spectroscopy, scanning Kerr-effect microscopy, and ferromagnetic resonance. Most of these studies are aimed at investigations of spin-wave eigenmodes of small magnetic elements, since they play an important role in ultrafast magnetization switching pro-cesses. On the other hand, radiation and propagation of spin waves also represent a crucial point for practical applications because they are associated with parasitic and/or desirable dynamic coupling phenomena in magnetic memory structures 6,7 and magnetic nano-oscillators. 8,9 Recent investi-gations on spin-wave radiation by micrometer-size spin-valve elements 6,7 have shown that the joint action of linear diffraction, caused by the anisotropic spin-wave dispersion, and of the lateral confinement results in complicated radia-tion and propagation characteristics of spin waves. Spin waves are intrinsically nonlinear objects. Recently nonlinear spin-wave effects occurring at large amplitudes of spin pre-cession became the subject of basic and applied studies see, e.g., Refs. 8–10. Therefore, investigation of the interplay between spin-wave diffraction and nonlinearities in confined magnetic structures is a next necessary step towards the un-derstanding of magnetic dynamics. In this letter we report on the experimental study of a joint action of spin-wave diffraction and nonlinearity on spin-wave beams in magnetic stripes, magnetized along their widths. We show that in the linear, low-amplitude propaga-tion regime the diffraction leads to a modulation of the width of a beam with the propagation distance. This effect can be considered as sequent spatial focusing and defocusing, caused by the in-plane anisotropy of spin-wave dispersion and by reflections at the side edges of the stripe. We find that with increasing amplitude of spin waves the linear diffraction is suppressed, which results in a significant weakening of the modulation of the beam width. Such stabilization is believed to be due to the suppression of linear focusing by the defo-cusing spin-wave nonlinearity. The experiments were performed on magnetic stripes with the width of 1 – 2 mm and the length of 40 mm prepared by chemical etching from epitaxial yttrium iron garnet YIG films with the thickness of 5.1 m. As indicated in Fig. 1, a static magnetic field H was applied in the plane of the stripe perpendicular to its axis, aligning the stripe magnetization along this direction. The value of the field was varied be-tween 600 and 1000 Oe. Spin waves, excited by a 50-m-wide microstrip transducer, propagated along the stripe, i.e., perpendicular to the static magnetization. Thus, the experimental geometry corresponds to surface or the so-called Damon-Eshbach 11 spin-wave modes. The excitation was performed by microwaves with the frequency F = 3.5– 5 GHz and the peak power in the range from 1 mW to 1 W. To avoid sample heating by intensive micro-wave radiation, microwave pulses with the pulse duration of 50 ns and the period of 1 s were used. The comparison of the results obtained in pulsed and continuous-wave modes for the excitation power of 1 mW showed their good coinci-dence. The detection of propagating spin waves was done by space-and time-resolved Brillouin light scattering spectros-copy in the forward scattering geometry. 10 This technique allows the visualisation of two-dimensional spatial distribu-tions of intensity of propagating spin waves in a very large range of their intensities from the thermal level to values far above the nonlinearity threshold.
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Vladislav E. Demidov
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ABSTRACT: We have studied experimentally the propagation of spin waves in microscopic transversally magnetized permalloy stripe waveguides with variable width. Spatially resolved measurement based on the microfocus Brillouin light-scattering spectroscopy allowed a direct observation of transformations of propagating transverse spin-wave modes in the region of the width transition. Our experiments show that due to the variation in the internal demagnetizing fields caused by the width variation, an effective control of the spin-wave propagation can be achieved. In particular, a splitting of a spin-wave beam into two independent beams or preferred excitation of certain transverse spin-wave modes can be realized.
Phys. Rev. B. 79(5).
